The invention relates to overlock sewing machines in general, and more particularly to improvements in mechanisms for severing threads in overlock sewing machines
Overlock sewing machines are used for overedging, for cutting and overedging, for seaming and overedging, for roll hemming, for overlocking of several material layers and cross seams and/or for a number of other special operations of utilitarian and/or decorative nature. For example, an overlock sewing machine can be used for overedging and simultaneous formation of a two-thread chain stitch next to the edge. A so-called safety stitch, for example, of the type SSa-2 according to stitch type 516 (US Federal Standard No. 751a) consists of an edge stitch (stitch type 504) and a parallel stitch (stitch type 401) of the aforementioned US Federal Standard No. 751a.
During sewing, the warp thread of the parallel stitch is guided by a hook or shuttle which is caused to oscillate transversely of the direction of stitching. The hook cooperates with thread guide means and with a thread looping unit which rotates in synchronism with movements of the hook. The guide means includes a substantially U-shaped carrier for a device which tensions the yarn, and the carrier has guide slots for the thread as well as an eyelet through which the thread passes to be deflected in a predetermined direction, namely toward the hook. The legs of the U-shaped carrier flank two rotary entraining and looping discs which are coaxial with but spaced apart from each other to provide room for a thread stripping device which is secured to the frame of the sewing machine.
A thread which is stored in the form of a cone, spool or bobbin advances from the respective source through the thread tensioning device and thereupon transversely of the direction of rotation of the looping discs, through the slots in the legs of the carrier, toward the eyelet and thence to the hook or shuttle. The discs are positioned with reference to the carrier in such a way that the thread is adjacent their peripheral surfaces. When the shaft which carries the discs is set in rotary motion, flats at the peripheries of the discs engage the thread between the legs of the carrier and pull the thus engaged thread toward the stripping device which segregates the freshly looped portion of the thread from the discs and enables the hook or shuttle to advance a length of thread toward the needle. The same operation is repeated during each cycle.
If the thread happens to break, it is still likely to be engaged by the looping discs but the loop which is formed by such discs cannot be separated by the stripping device because the leader of the broken thread is no longer under tension. The broken thread is simply convoluted onto the shaft which drives the looping discs. This necessitates a lengthy interruption of operation of the sewing machine because the convolutions are not readily removable. In fact, the package of convoluted thread can cause damage to the carrier and/or to the looping discs.